Conventional high-strength steel wire rods or intermediate products of steel wire rods are generally produced by two methods. In the first of the two methods, a heat treatment process using a solder pot is performed once or twice on a steel wire rod between a hot rolling process and a cold drawing process, so as to increase the strength of the steel wire rod. This method is widely used to produce tire bead wires, and saw wires for cutting semiconductor wafers.
In the second of the two methods, a steel wire rod produced through a hot-rolling process is processed through quenching and tempering processes so as to have a desired degree of tensile strength.
The former method is usually used for producing narrow steel wire rods (having a diameter of about 0.1 mm to about 5 mm). That is, the former method is not suitable for producing steel wire rods for structural mechanical parts. Therefore, the latter method in which a desired degree of strength is obtained by heat treatments is usually used to produce steel wire rods for structural mechanical parts. Steel wire rods produced using quenching and tempering processes have mechanical characteristics determined by the heat treatments and alloying elements added thereto, and thus, such steel wire rods may be formed to have high tensile strength and ductility. However, large amounts of relatively expensive elements (such as molybdenum (Mo), vanadium (V), chromium (Cr), or nickel (Ni)) are added to the steel wire rods to guarantee the stability of the steel wire rods in terms of factors such as resistance to hydrogen delayed fracture, and thus manufacturing costs thereof may be increased.
Recently, automobiles have been required to be relatively lightweight while having high performance and energy saving features, and thus, parts such as bolts for driving units or engines are required to have high strength. Current high-strength bolts are formed of high-strength wire rods having a strength of about 1200 MPa and are formed through quenching and tempering processes by using alloy steels such as SCM435 or SCM440. However, since hydrogen delayed fracture may easily occur in steel wire rods having a tensile strength of 1200 MPa or greater, the usage of such steel wire rods is limited.
Most high-strength steel wire rods are formed of quenched and tempered steels by performing a hot-rolling process in order to form hot-rolled wire rods (intermediate products), and performing reheating, quenching and tempering processes on the hot-rolled wire rods. However, non-quenched and tempered steels may be used. Non-quenched and tempered steels may have levels of ductility and strength similar to those of heat-treated (quenched and tempered) steels even in the case that they are manufactured without performing a heat treatment process after a hot-rolling process. In Korea and Japan, such steels are known as “non-quenched and tempered steels.” However, in countries such as Britain and the United States, such steels are called “non-heat-treated steels” because no heat treatment is performed thereon, or “micro-alloyed steels” because small amounts of alloying elements are added thereto.
Generally, processes for manufacturing steel wire rods using quenched and tempered steels include a hot-rolling process, a cold drawing process, a spheroidizing heat treatment process, a cold drawing process, a cold forging process, a quenching process, and a tempering process; while processes for manufacturing steel wire rods using non-quenched and tempered steels include a hot-rolling process, a cold drawing process, and a cold forging process. Therefore, steel wire rods formed of non-quenched and tempered steels are more economical owing to the low manufacturing costs thereof.
As described above, non-quenched and tempered steels are economical because heat treatment processes are omitted. In addition, since final quenching and tempering processes are not performed, defects such as bending caused by heat treatments are not present, and desired degrees of straightness are obtained. Therefore, many products are manufactured using non-quenched and tempered steels. However, due to the omission of heat treatments and the repetition of cold forming, the ductility of products is gradually decreased as processes proceed, even though the strength of products is increased.
A technique relating to this is disclosed in Patent Document 1. In Patent Document 1 (Japanese Patent Application Laid-open Publication No.: 2012-041587), a special steel having one or both of pro-eutectoid ferrite and bainite microstructures is proposed, and a quenched and tempered steel wire rod having a tempered martensite microstructure as a final microstructure is formed by heat-treating the special steel. According to Patent Document 1, a steel wire rod is manufactured by heating a slab having an alloying composition of carbon (C): 0.35 wt % to 0.85 wt %, silicon (Si): 0.05 wt % to 2.0 wt %, manganese (Mn): 0.20 wt % to 1.0 wt %, chromium (Cr): 0.02 wt % to 1.0 wt %, nickel (Ni): 0.02 wt % to 0.5 wt %, titanium (Ti): 0.002 wt % to 0.05 wt %, vanadium (V): 0.01 wt % to 0.20 wt %, niobium (Nb): 0.005 wt % to 0.1 wt %, and boron (B): 0.0001 wt % to 0.0060 wt %; rolling the slab to form a wire rod and cooling the wire rod; heating the wire rod to 750° C. to 950° C.; and processing the wire rod in a salt bath at a constant temperature of 400° C. to 600° C. Finally, the wire rod has a degree of strength within the range of 1500 MPa to 2000 MPa. According to the technique disclosed in Patent Document 1, a final degree of strength is obtained through a heat treatment process. However, the technique is not useful because of the complex composition of the wire rod and the increase in manufacturing costs due to the heat treatment process.
Patent Document 2 (Japanese Patent Application Laid-open Publication No.: 2005-002413) discloses a steel wire rod in which hypereutectoid pearlite having a pearlite interlayer gap of 200 μm to 300 μm is formed. The final strength of the steel wire rod is 4000 MPa to 5000 MPa. The steel wire rod is manufactured by producing an intermediate product through heating, wire rolling, and cooling, and performing first and second cold drawing processes and a lead patenting process on the intermediate product. The steel wire rod has an alloying composition of carbon (C): 0.8 wt % to 1.1 wt %, silicon (Si): 0.1 wt % to 1.0 wt %, manganese (Mn): 0.1 wt % to 1.0 wt %, chromium (Cr): 0.6 wt % or less, and boron (B): 0.005 wt % or less. However, the steel wire rod requires a drawing process up to about 0.18 mm, and thus, the steel wire rod is not suitable for use as a structural steel wire rod.
Patent Document 3 (Japanese Patent Application Laid-open Publication No.: 2011-225990) discloses a steel wire rod for a drawing process. The steel wire rod has a pearlite microstructure having 100 or fewer BN-based compounds and is processed through a cold forming process so that the steel wire rod may have a tensile strength of about 3500 MPa. The steel wire rod is manufactured by forming an intermediate product through heating to 100° C. to 1300° C., wire rolling, and cooling from 850° C. to 950° C. to 600° C. at a rate of 35° C./s. Then, a hot-rolling process, first and second cold drawing processes, and a lead patenting process are performed on the intermediate product to form the steel wire rod. Main alloying elements of the steel wire rod include carbon (C): 0.70 wt % to 1.2 wt %, silicon (Si): 0.1 wt % to 1.5 wt %, manganese (Mn): 0.1 wt % to 1.5 wt %, copper (Cu): 0.25 wt % or less, chromium (Cr): 1.0 wt % or less, boron (B): 0.0005 wt % to 0.001 wt %, and nitrogen (N): 0.002 wt % to 0.005 wt %. However, the steel wire rod requires a drawing process up to about 0.18 mm, and thus the steel wire rod is not suitable for use as a structural steel wire rod.